1. Field of the Invention
The present invention relates to doctor blades and, more particularly, is directed towards doctor blades for papermaking machines.
2. Description of the Prior Art
Doctor blades contact the surface of rotating rolls of papermaking machines for various purposes, such as roll cleaning and sheet shedding. Conventional doctor blades have been constructed either of a hard material, such as metal, or a soft material, such as plastic. Hard doctor blades do not conform readily to the contour of the moving surface of the roll against which they are pressed. On the other hand, soft doctor blades wear out too quickly with the result that they have to be reground and replaced frequently.
Generally, metal blades are made of steel, stainless steel, nickle, bronze, etc. Steel is preferred because it is inexpensive and it has high stiffness and good wear characteristics. The drawback with steel blades is that they are susceptible to corrosion in wet environments. In addition, steel blades cause the rolls to wear prematurely.
Plastic doctor blades are employed in places where steel blades can not be used. Plastic blades having the same thickness as comparable steel blades do not have sufficient stiffness in the machine direction. In order to compensate for this deficiency, plastic blades are made relatively thick, for example 1/16" to 1/4". Thick blades suffer from the disadvantage that additional power is required to drive the roll against which the blade is pressed, the power required being proportional to blade thickness. More particularly, since the surface area of thick blades is greater than that of thin blades, more force is required to press the thick plastic blade against the roll in order to achieve the desired loading pressure. This in turn results in higher torques and greater power losses. Also, the doctoring edge of the thick plastic blade tends to deflect outwardly with the results that the material being doctored starts to work its way between the blade and the surface being doctored. Furthermore, plastic doctor blades normally begin to degrade, that is, lose stiffness or develop blisters when operating at temperatures in the range of 200.degree. F.-300.degree. F.
U.S. Pat. No. 2,767,529 shows a doctor blade composed of alternate frangible and wear resisting laminations of mildly abrasive and non-abrasive materials, respectively. U.S. Pat. Nos. 3,768,760 and 4,173,670 respectively show an airfoil with graphite fibers and a tubular element with graphite fibers.